These options are not mutually exclusive. For example, the same application
can store information in a Cloud SQL database, and also store files in a Cloud
Storage bucket.

Third-party databases

You can also choose from a variety of third-party databases to use with your
applications such as Redis, MongoDB, PostgreSQL, Cassandra, and Hadoop.
Any third-party database supported by your language can be used with your
App Engine instance, provided the two can connect to each other.